1. Field of the Invention
The present invention relates to a method of measuring the roundness of a ball end mill.
2. Description of the Background Art
In metal mold machining, the formation of free curved surfaces is normally performed by machining methods utilizing ball end mills because, when machining a curved surface, a cutting point is present on the normal to a machined surface, and ordinary angle end mills therefore, can not machine along such a shape.
In finish machining, it is especially necessary to form a desired curved surface, which requires high precision ball end mills.
As shown in FIG. 11(a), when a dish-shaped circular arc of a work W is cut by a ball end mill T whose cutting edge shape is far from a sphere, it is impossible to finish a machined surface into a desired shape. A result is shown in FIG. 11(b).
In general, the sphericity of ball end mills depends on precision at the time of purchase. In actual machining, it is difficult to obtain precise roundness because precision is lowered due such ordinary factors as the tool holders deflection and main spindle, the wearing-out of cutting edges, and the like.
Tool manufacturers often use visual means to confirm precision while rotating a tool by using a tool microscope with a projector. As a result, visual observation measuring error is added to error from cutting edge grinding and from removal for measurement. The guarantee precision for general tools is about 0.1 mm, while that for high precision tools is about 0.03 mm. It is therefore evident that the total error when these tools are used for machining makes it impossible to achieve the required precision.
Normally, in machining employing ball end mills, the diameter of tools is altered depending on machining shape in a range from about 50 mm to about 3 mm in terms of machining efficiency and tool rigidity. In finish machining, the previous tool leaves a machining line, causing a partial concave-shape.
As described above, in high precision machining employing ball end mills, precise sphericity is an essential property desired in balls (cutting faces). Devices for tool inspection that measure the sphericity of cutting edges have recently been developed. As shown in FIG. 12, Japanese Patent Publication No. Hei. 7-49955 (1995) discloses a device for tool inspection comprising a work head 101 that horizontally fixes a base of a ball end mill T and carries it rotatably and a measuring probe 102 that measures the shape of a cutting part of the ball end mill. The measuring probe 102 is pivotable horizontally in the center of a pivot 103 on the horizontal and also can advance or retreat to the pivot 103 on the horizontal. The ball end mill T carried by the work head 101 is rotatable around the X-axis crossing at a right angle with the pivot 103.
The above publication also discloses a measuring method of ball end mills utilizing the aforesaid device for tool inspection. In this method, in the process of preparation the measuring probe 102 is allowed to come into contact with the tip of the ball end mill T to calculate the distance from the center of the pivot 103 to the tip of the cutting edge. The rotated angular position of the measuring probe 102 and the horizontal plane are calculated. Thereafter, the measuring probe 102 is turned to a position where it crosses at a right angle to the X-axis on the horizontal, thereby measuring the diameter of a ball to obtain its radius. Based on the results, the ball end mill is positioned by moving along X-axis direction so that the pivot is located one radius away from the tip of the ball end mill T. The distance from the ball end mill T to the tip of the measuring probe 102 is then measured.
Both the microscope for tool inspection in the prior art and the device for tool inspection disclosed in the above publication, however, introduce error due to attachment for inspection and thus fail to obtain the actual value of roundness precision. For tools users, roundness as measured when fixed to a main spindle is of importance and it is a serious defect that the ultimate roundness, obtained by adding the deflections of a main spindle and a tool holder to the precision of a tool itself, cannot be measured.